fix: Save draft message in Chat Form when adding System Prompt from new chat view

tools/server/webui/src/lib/components/app/chat/ChatForm/ChatForm.svelte

@@ -12,7 +12,7 @@
 		onFileUpload?: (files: File[]) => void;
 		onSend?: (message: string, files?: ChatUploadedFile[]) => Promise<boolean>;
 		onStop?: () => void;
-		onSystemPromptAdd?: () => void;
+		onSystemPromptAdd?: (draft: { message: string; files: ChatUploadedFile[] }) => void;
 		showHelperText?: boolean;
 		uploadedFiles?: ChatUploadedFile[];
 	}
@@ -32,8 +32,21 @@
 	}: Props = $props();
 
 	let inputAreaRef: ChatFormInputArea | undefined = $state(undefined);
-	let message = $state('');
+	let message = $state(initialMessage);
 	let previousIsLoading = $state(isLoading);
+	let previousInitialMessage = $state(initialMessage);
+
+	// Sync message when initialMessage prop changes (e.g., after draft restoration)
+	$effect(() => {
+		if (initialMessage !== previousInitialMessage) {
+			message = initialMessage;
+			previousInitialMessage = initialMessage;
+		}
+	});
+
+	function handleSystemPromptClick() {
+		onSystemPromptAdd?.({ message, files: uploadedFiles });
+	}
 
 	let hasLoadingAttachments = $derived(uploadedFiles.some((f) => f.isLoading));
 
@@ -101,7 +114,7 @@
 		onFilesAdd={handleFilesAdd}
 		{onStop}
 		onSubmit={handleSubmit}
-		onSystemPromptClick={onSystemPromptAdd}
+		onSystemPromptClick={handleSystemPromptClick}
 		onUploadedFileRemove={handleUploadedFileRemove}
 	/>
 </div>

tools/server/webui/src/lib/components/app/chat/ChatForm/ChatFormInputArea.svelte

@@ -39,7 +39,7 @@
 		onMcpPromptArgumentsChange?: (fileId: string, args: Record<string, string>) => void;
 		onStop?: () => void;
 		onSubmit?: () => void;
-		onSystemPromptClick?: () => void;
+		onSystemPromptClick?: (draft: { message: string; files: ChatUploadedFile[] }) => void;
 		onUploadedFileRemove?: (fileId: string) => void;
 		onUploadedFilesChange?: (files: ChatUploadedFile[]) => void;
 		onValueChange?: (value: string) => void;
@@ -422,7 +422,7 @@
 				onFileUpload={handleFileUpload}
 				onMicClick={handleMicClick}
 				{onStop}
-				{onSystemPromptClick}
+				onSystemPromptClick={() => onSystemPromptClick?.({ message: value, files: uploadedFiles })}
 				onMcpPromptClick={showMcpPromptButton ? () => (isPromptPickerOpen = true) : undefined}
 			/>
 		</div>

tools/server/webui/src/lib/components/app/chat/ChatScreen/ChatScreen.svelte

@@ -400,7 +400,7 @@
 					onFileUpload={handleFileUpload}
 					onSend={handleSendMessage}
 					onStop={() => chatStore.stopGeneration()}
-					onSystemPromptAdd={() => chatStore.addSystemPrompt()}
+					onSystemPromptAdd={handleSystemPromptAdd}
 					showHelperText={false}
 					bind:uploadedFiles
 				/>
@@ -460,7 +460,7 @@
 					onFileUpload={handleFileUpload}
 					onSend={handleSendMessage}
 					onStop={() => chatStore.stopGeneration()}
-					onSystemPromptAdd={() => chatStore.addSystemPrompt()}
+					onSystemPromptAdd={handleSystemPromptAdd}
 					showHelperText={true}
 					bind:uploadedFiles
 				/>

tools/server/webui/src/lib/stores/chat.svelte.ts

@@ -41,6 +41,10 @@ class ChatStore {
 	private addFilesHandler: ((files: File[]) => void) | null = $state(null);
 	pendingEditMessageId = $state<string | null>(null);
 
+	// Draft preservation for navigation (e.g., when adding system prompt from welcome page)
+	private _pendingDraftMessage = $state<string>('');
+	private _pendingDraftFiles = $state<ChatUploadedFile[]>([]);
+
 	constructor() {
 		if (browser) {
 			chatClient.setStoreCallbacks({
@@ -219,6 +223,31 @@ class ChatStore {
 		this.pendingEditMessageId = null;
 	}
 
+	savePendingDraft(message: string, files: ChatUploadedFile[]): void {
+		this._pendingDraftMessage = message;
+		this._pendingDraftFiles = [...files];
+	}
+
+	consumePendingDraft(): { message: string; files: ChatUploadedFile[] } | null {
+		if (!this._pendingDraftMessage && this._pendingDraftFiles.length === 0) {
+			return null;
+		}
+
+		const draft = {
+			message: this._pendingDraftMessage,
+			files: [...this._pendingDraftFiles]
+		};
+
+		this._pendingDraftMessage = '';
+		this._pendingDraftFiles = [];
+
+		return draft;
+	}
+
+	hasPendingDraft(): boolean {
+		return Boolean(this._pendingDraftMessage) || this._pendingDraftFiles.length > 0;
+	}
+
 	getAllLoadingChats(): string[] {
 		return Array.from(this.chatLoadingStates.keys());
 	}